Solar cell and method for manufacturing the same

ABSTRACT

A solar cell and a method for manufacturing the same are disclosed. The solar cell may include a substrate, an emitter layer positioned at a first surface of the substrate, a first anti-reflection layer that is positioned on a surface of the emitter layer and may include a plurality of first contact lines exposing a portion of the emitter layer, a first electrode that is electrically connected to the emitter layer exposed through the plurality of first contact lines and may include a plating layer directly contacting the emitter layer, and a second electrode positioned on a second surface of the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending application Ser. No.13/640,713 filed on Oct. 11, 2012, which is the national phase of PCTInternational Application No. PCT/KR2010/007460 filed on Oct. 28, 2010,and which claims priority to Application No. 10-2010-0086464 filed inthe Republic of Korea on Sep. 3, 2010. The entire contents of all of theabove applications are hereby incorporated by reference.

BACKGROUND

Field of the Disclosure

Exemplary embodiments of the invention relate to a solar cell and amethod for manufacturing the same.

Description of the Related Art

The solar power generation of converting light energy into electricenergy using a photoelectric transformation effect has been widely usedas a method for obtaining eco-friendly energy. A solar power generationsystem using a plurality of solar cell panels has been installed inhouses due to improvement of photoelectric transformation efficiency ofsolar cells.

The solar cell generally includes a substrate and an emitter layer thatforms a p-n junction along with the substrate, thereby generating acurrent from light incident on the solar cell through one surface of thesubstrate.

Because light is generally incident on the solar cell through only onesurface of the substrate, current transformation efficiency of the solarcell is low. Accordingly, a double-sided light receiving solar cell, inwhich light is incident on the solar cell through both surfaces of thesubstrate, has been recently developed.

SUMMARY

In one aspect, there is a solar cell including a substrate including auniform first surface, an emitter layer positioned at a first surface ofthe substrate, a first anti-reflection layer positioned on a surface ofthe emitter layer, the first anti-reflection layer including a pluralityof first contact lines exposing a portion of the emitter layer, a firstelectrode electrically connected to the emitter layer exposed throughthe plurality of first contact lines, the first electrode including aplating layer directly contacting the emitter layer, and a secondelectrode positioned on a second surface of the substrate.

Each of the plurality of first contact lines has a width of about 20 μmto 60 μm, and a plane area of each of the plurality of first contactlines is about 2% to 6% of a plane area of the emitter layer. The firstelectrode has a thickness of about 20 μm to 50 μm. As a result, thefirst electrode has a narrow width and a high aspect ratio, for example,an aspect ratio of about 0.83 to 1.

The first surface and the second surface of the substrate may beuniformly textured to form a first textured surface and a secondtextured surface, respectively.

The first anti-reflection layer may include a silicon nitride layer anda silicon oxide layer or an aluminum oxide layer positioned between theemitter layer and the silicon nitride layer. The substrate may be formedof an n-type silicon wafer doped with phosphorus (P).

The solar cell may further include a back surface field layer positionedat the second surface of the substrate and a second anti-reflectionlayer positioned on a surface of the back surface field layer on whichthe second electrode is not positioned.

The first electrode and the second electrode may be formed of differentmaterials. For example, a plating layer that may be used to form thefirst electrode may include a metal seed layer, that directly contactsthe emitter layer and contains nickel, and at least one conductivelayer, that is positioned on the metal seed layer and contains at leastone selected from the group consisting of copper (Cu), silver (Ag),aluminum (Al), tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold(Au), and a combination thereof. The second electrode may be formed ofsilver (Ag).

A width of the second electrode may be greater than a width of the firstelectrode. The second anti-reflection layer may include a siliconnitride layer.

A method for manufacturing the solar cell having the above-describedconfiguration may include texturing the first surface and the secondsurface of the substrate to form the first textured surface and thesecond textured surface, respectively, forming the emitter layer at thefirst surface of the substrate and forming the back surface field layerat the second surface of the substrate, forming the firstanti-reflection layer on the surface of the emitter layer and formingthe second anti-reflection layer on the surface of the back surfacefield layer, forming the plurality of first contact lines on the firstanti-reflection layer, forming the second electrode on the surface ofthe second anti-reflection layer, and forming the first electrode on theplurality of first contact lines, wherein the first electrode and thesecond electrode are formed of different materials.

The process for forming of the plurality of first contact lines may usea wet etching process or a dry etching process using a laser. Morespecifically, the forming of the plurality of first contact lines mayinclude etching the first anti-reflection layer using the dry etchingprocess using the laser and removing a damaged layer of the emitterlayer generated by the laser using the wet etching process.

The process for forming the second electrode may include printing aconductive paste obtained by mixing silver (Ag) with a glass frit on thesurface of the second anti-reflection layer and drying and firing theconductive paste. The forming of the first electrode may include forminga metal seed layer directly contacting the emitter layer and forming atleast one conductive layer on the metal seed layer.

Further, the second anti-reflection layer may include a plurality ofsecond contact lines exposing a portion of the back surface field layer.Each of the plurality of second contact lines may have a width of about40 μm to 100 μm. A plane area of each of the plurality of second contactlines may be about 5% to 15% of a plane area of the back surface fieldlayer.

The second electrode may include a metal seed layer directly contactingthe back surface field layer, which is exposed through the plurality ofsecond contact lines, and at least one conductive layer positioned on asurface of the metal seed layer. The first and second electrodes mayhave the same structure.

For example, the metal seed layer of each of the first and secondelectrodes may contain nickel. The at least one conductive layer of eachof the first and second electrodes may contain at least one selectedfrom the group consisting of copper (Cu), silver (Ag), aluminum (Al),tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and acombination thereof.

A method for manufacturing the solar cell having the above-describedconfiguration may include texturing the first surface and the secondsurface of the substrate to form the first textured surface and thesecond textured surface, respectively, forming the emitter layer at thefront surface of the substrate and forming the back surface field layerat the second surface of the substrate, forming the firstanti-reflection layer on the surface of the emitter layer and formingthe second anti-reflection layer on the surface of the back surfacefield layer, forming the plurality of first contact lines on the firstanti-reflection layer and forming the plurality of second contact lineson the second anti-reflection layer, and forming the first electrode onthe emitter layer exposed through the plurality of first contact linesand forming the second electrode on the back surface field layer exposedthrough the plurality of second contact lines, wherein the firstelectrode and the second electrode are formed of the same material.

The process for forming of the plurality of first and second contactlines may use a wet etching process or a dry etching process using alaser. More specifically, the forming of the plurality of first andsecond contact lines may include etching the first anti-reflection layerand the second anti-reflection layer using a dry etching process using alaser and removing a damaged layer of the emitter layer and a damagedlayer of the back surface field layer, that are generated by the laser,using a wet etching process.

The forming of the first and second electrodes may include forming ametal seed layer directly contacting the emitter layer or the backsurface field layer and forming at least one conductive layer on themetal seed layer.

In the solar cell having the above-described characteristics, becauseboth the first surface and the second surface of the substrate are thetextured surfaces and the first and second anti-reflection layersserving as passivation layers are respectively positioned on the firstsurface and the second surface of the substrate, the solar cell may beused to generate a current by allowing light, that is incident on thefirst surface of the substrate and then is transmitted by the substrate,to be again incident on the second surface of the substrate.Accordingly, the efficiency of the solar cell according to the exemplaryembodiment of the invention may increase, as compared to a solar cellgenerating the current using only light incident on one surface of thesubstrate.

Further, because the first electrode may be formed using a platingelectrode, the width of the first electrode may be less than a width ofa related art conductive paste used as an electrode material, and theaspect ratio of the first electrode may increase. Therefore, lightincident area may increase, and the efficiency of the solar cell mayalso increase.

Further, when a surface resistance of the emitter layer increases,contact between the first electrode and the emitter layer may besmoothly maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a schematic cross-sectional view of a solar cell according toan exemplary embodiment of the invention;

FIG. 2 is an enlarged cross-sectional view of a portion of the solarcell shown in FIG. 1;

FIGS. 3 to 5 are cross-sectional views sequentially illustrating anexemplary method for manufacturing the solar cell shown in FIG. 1;

FIG. 6 is a cross-sectional view sequentially illustrating an exemplarymethod for manufacturing a substrate of the solar cell shown in FIG. 3;

FIG. 7 is a schematic cross-sectional view of a solar cell according toanother exemplary embodiment of the invention; and

FIGS. 8 and 9 are cross-sectional views sequentially illustrating anexemplary method for manufacturing the solar cell shown in FIG. 7.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will be described more fully hereinafter with reference tothe accompanying drawings, in which example embodiments of theinventions are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. Like-reference numerals designatelike-elements throughout the specification. It will be understood thatwhen an element such as a layer, film, region, or substrate is referredto as being “on” another element, it can be directly on the otherelement or intervening elements may also be present. In contrast, whenan element is referred to as being “directly on” another element, thereare no intervening elements present. Further, it will be understood thatwhen an element such as a layer, film, region, or substrate is referredto as being “entirely” on another element, it may be on the entiresurface of the other element and may not be on a portion of an edge ofthe other element.

Reference will now be made in detail to embodiments of the invention,examples of which are illustrated in the accompanying drawings.

FIG. 1 is a schematic cross-sectional view of a solar cell according toan exemplary embodiment of the invention. FIG. 2 is an enlargedcross-sectional view of a portion of the solar cell shown in FIG. 1.FIGS. 3 to 5 are cross-sectional views sequentially illustrating anexemplary method for manufacturing the solar cell shown in FIG. 1. FIG.6 is a cross-sectional view sequentially illustrating an exemplarymethod for manufacturing a substrate of the solar cell shown in FIG. 3

A solar cell according to an exemplary embodiment of the inventionincludes a substrate 110, an emitter layer 120 positioned at onesurface, for example, a front surface of the substrate 110, a firstanti-reflection layer 130 positioned on the emitter layer 120, aplurality of first electrodes 140 positioned on the emitter layer 120 onwhich the first anti-reflection layer 130 is not positioned, a backsurface field (BSF) layer 150 positioned at a back surface of thesubstrate 110, a second anti-reflection layer 160 positioned on a backsurface of the back surface field layer 150, and a plurality of secondelectrodes 170 positioned on the back surface of the back surface fieldlayer 150 on which the second anti-reflection layer 160 is notpositioned.

The substrate 110 may be formed of a silicon wafer of a first conductivetype, for example, an n-type, though not required. Silicon used in thesubstrate 110 may be crystalline silicon, such as single crystal siliconand polycrystalline silicon, or amorphous silicon. When the substrate110 is of the n-type, the substrate 110 contains impurities of a group Velement such as phosphorus (P), arsenic (As), and antimony (Sb).

Alternatively, the substrate 110 may be of a p-type and/or be formed ofother semiconductor materials other than silicon. When the substrate 110is of the p-type, the substrate 110 may contain impurities of a groupIII element such as boron (B), gallium (Ga), and indium (In).

As shown in FIG. 6, the surface of the substrate 110 may be uniformlytextured to form a textured surface corresponding to an uneven surfaceor having uneven characteristics. More specifically, the substrate 110has a first textured surface 111 corresponding to the front surface inwhich the emitter layer 120 is positioned and a second textured surface113 corresponding to the back surface at which the back surface fieldlayer 150 is positioned.

The emitter layer 120 positioned at the first textured surface 111 ofthe substrate 110 is an impurity region of a second conductive type (forexample, a p-type) opposite the first conductive type of the substrate110 and forms a p-n junction along with the substrate 110.

A plurality of electron-hole pairs produced by light incident on thesubstrate 110 are separated into electrons and holes by a built-inpotential difference resulting from the p-n junction between thesubstrate 110 and the emitter layer 120. The separated electrons move tothe n-type semiconductor, and the separated holes move to the p-typesemiconductor. When the substrate 110 is of the n-type and the emitterlayer 120 is of the p-type, the separated electrons and the separatedholes move to the substrate 110 and the emitter layer 120, respectively.Accordingly, the electrons become major carriers in the substrate 110,and the holes become major carriers in the emitter layer 120.

When the emitter layer 120 is of the p-type, the emitter layer 120 maybe formed by doping the substrate 110 with impurities of a group IIIelement such as B, Ga, and In.

Alternatively, when the substrate 110 is of the p-type, the emitterlayer 120 is of the n-type. In this case, the separated holes move tothe substrate 110, and the separated electrons move to the emitter layer120. When the emitter layer 120 is of the n-type, the emitter layer 120may be formed by doping the substrate 110 with impurities of a group Velement such as P, As, and Sb.

As shown in FIGS. 3-5, the first anti-reflection layer 130 on theemitter layer 120 in the front surface of the substrate 110 includes asilicon nitride (SiNx:H) layer 131 and an aluminum oxide (AlOx) layer133 between the emitter layer 120 and the silicon nitride layer 131. Thefirst anti-reflection layer 130 reduces reflectance of light incidentthrough the front surface of the substrate 110 and increases selectivityof a predetermined wavelength band, thereby increasing the efficiency ofthe solar cell.

In this embodiment the aluminum oxide layer 133 has a refractive indexof about 1.55 to 1.7 and a thickness equal to or less than about 50 nm,and the silicon nitride layer 131 has a refractive index of about 1.9 to2.3 and a thickness of about 50 nm to 100 nm, so as to minimize thelight reflectance in the first anti-reflection layer 130.

It could be seen from an experiment conducted by the present inventorsthat the light reflectance in the first anti-reflection layer 130 wasminimized when the first anti-reflection layer 130 has a double-layeredstructure including the silicon nitride layer 131 and the aluminum oxidelayer 133, each of which is within the above refractive index andthickness ranges.

A silicon oxide (SiOx:H) layer may be used instead of the aluminum oxidelayer 133.

The first anti-reflection layer 130 may include a plurality of firstcontact lines CL1 exposing a portion of the emitter layer 120. The firstelectrodes 140 (see FIG. 1) may be formed on the emitter layer 120exposed through the first contact lines CL1.

In this embodiment, the first contact line CL1 has a width W1 of about20 μm to 60 μm, and a plane area of the first contact line CL1 is about2% to 6% of a plane area of the emitter layer 120, so that the firstelectrode 140 has a narrow width and a high aspect ratio.

When the first contact line CL1 has the width W1, the first electrode140 may be formed to have a thickness T1 of about 20 μm to 50 μm using aplating process.

FIG. 1 shows that the thickness T1 of the first electrode 140 indicatesa distance from a convex portion of the emitter layer 120 to an uppersurface of the first electrode 140. Because a distance from a concaveportion to the convex portion of the emitter layer 120 is much shorterthan the thickness T1 of the first electrode 140, it does not matterthat the thickness T1 of the first electrode 140 is represented by thedistance from the convex portion of the emitter layer 120 to the uppersurface of the first electrode 140.

According to the above-described structure, the first electrode 140 hasa high aspect ratio of about 0.83 to 1.

The first electrodes 140 formed on the emitter layer 120 exposed throughthe first contact line CL1 are electrically and physically connected tothe emitter layer 120. The first electrodes 140 extend substantiallyparallel to one another in a fixed direction.

The first electrodes 140 collect carriers (for example, holes) moving tothe emitter layer 120. In the exemplary embodiment of the invention, thefirst electrodes 140 may be finger electrodes. Alternatively, each firstelectrode 140 may be a finger electrode current collector or both afinger electrode and a finger electrode current collector.

As shown in FIG. 2, in the exemplary embodiment of the invention, thefirst electrode 140 may be formed of a plating layer. The plating layermay include at least one of a metal seed layer 141, a diffusion barrierlayer 142, and a conductive layer 143 that may be sequentially formed onthe emitter layer 120, if there is more than one layer in the platinglayer.

The metal seed layer 141 may be formed of a material containing nickel,for example, nickel silicide (including Ni₂Si, NiSi, NiSi₂, etc.) andhas a thickness of about 50 nm to 200 nm.

When the thickness of the metal seed layer 141 is less than 50 nm, ahigh resistance is obtained and it is difficult to form a uniform metalseed layer 141. Thus, it is difficult to achieve uniformity in asubsequent process, i.e., in a plating process of the diffusion barrierlayer 142. When the thickness of the metal seed layer 141 is greaterthan 200 nm, the metal seed layer 141 is distributed to silicon at aconstant rate in a thermal process to form a nickel silicide layer.Thus, a shunt leakage current may occur because of the distribution ofnickel.

The diffusion barrier layer 142 on the metal seed layer 141 preventsjunction degradation generated when a formation material of theconductive layer 143 is diffused into a silicon interface through themetal seed layer 141. The diffusion barrier layer 142 includes a nickellayer having a thickness of about 5 μm to 15 μm.

The conductive layer 143 on the diffusion barrier layer 142 is formed ofat least one conductive metal material. Examples of the at least oneconductive metal material include at least one selected from the groupconsisting of nickel (Ni), copper (Cu), silver (Ag), aluminum (Al), tin(Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and acombination thereof. Other materials may be used.

In the exemplary embodiment of the invention, the conductive layer 143may include a copper layer 143 a. The copper layer 143 a substantiallyserves as an electrical wire and has a thickness of about 10 μm to 30μm. However, it is known that copper easily oxidizes in the air. Also,it is difficult to directly solder an interconnector, for example, aribbon (not shown) for electrically connecting the adjacent solar cellsto the copper layer 143 a in module processing. Thus, when theconductive layer 143 includes the copper layer 143 a, the conductivelayer 143 may further include a tin layer 143 b that preventsoxidization of copper and may be used to smoothly perform a solderingprocess of the ribbon. The tin layer 143 b on the copper layer 143 a hasa thickness of about 5 μm to 15 μm.

When the conductive layer 143 includes a metal material other than thecopper layer 143 a, the tin layer 143 b may be omitted if the conductivelayer does not easily oxidize in the air and can be used to smoothlyperform the soldering process of the ribbon.

When the first electrode 140 is a finger electrode, a current collectorfor collecting carriers moving to the finger electrode may be furtherformed on the front surface of the substrate 110. The current collectormay be formed using a conductive electrode in the same manner as thefirst electrode 140. Also, the current collector may be formed byprinting, drying, and firing a conductive paste containing a conductivematerial, unlike the first electrode 140.

The second electrodes 170 on the back surface of the substrate 110collect carriers (for example, electrons) moving to the substrate 110and output the carriers to an external device. In the exemplaryembodiment of the invention, the second electrodes 170 may be fingerelectrodes. Alternatively, each second electrode 170 may be a fingerelectrode current collector or both a finger electrode and a fingerelectrode current collector.

The second electrodes 170 may be formed of at least one conductivematerial selected from the group consisting of aluminum (Al), nickel(Ni), copper (Cu), silver (Ag), tin (Sn), zinc (Zn), indium (In),titanium (Ti), gold (Au), and a combination thereof. In the exemplaryembodiment of the invention, the second electrodes 170 are formed ofsilver (Ag).

The second electrode 170 may have a width W2 greater than a width of thefirst electrode 140 (i.e., the width W1 of the first contact line CL1),and a pitch between the second electrodes 170 may be less than a pitchbetween the first electrodes 140, so that line resistance is reduced.The pitch between the electrodes indicates a distance between theadjacent electrodes.

The back surface field layer 150, electrically and physically connectedto the second electrode 170, is positioned at the entire back surface ofthe substrate 110. The back surface field layer 150 is a region (forexample, n⁺-type region) that is more heavily doped than the substrate110 with the same conductive type impurities as the substrate 110.

The movement of holes to the back surface of the substrate 110 may beprevented or reduced by a potential barrier resulting from a differencebetween impurity concentrations of the substrate 110 and the backsurface field layer 150. Hence, a recombination and/or a disappearanceof electrons and holes around the surface of the substrate 110 may beprevented or reduced.

The second anti-reflection layer 160 may be positioned on the backsurface of the back surface field layer 150 on which the secondelectrodes 170 are not positioned. The second anti-reflection layer 160may be formed using a silicon nitride (SiNx:H) layer.

The solar cell having the above-described structure according to theexemplary embodiment of the invention may serve as a double-sided lightreceiving solar cell, and an operation of the solar cell is describedbelow.

When light irradiated onto the solar cell is incident on the substrate110 through the emitter layer 120 and/or the back surface field layer150, a plurality of electron-hole pairs are generated in the substrate110 by the light energy. In this case, because the front surface and theback surface of the substrate 110 may be the first textured surface 111and the second textured surface 113, respectively, a light reflectancein each of the front surface and the back surface of the substrate 110is reduced. Further, because both a light incident operation and a lightreflection operation may be performed on each of the first and secondtextured surfaces 111 and 113 of the substrate 110, light may beconfined in the solar cell. Hence, light absorption increases, and theefficiency of the solar cell is improved. In addition, because areflection loss of the light incident on the substrate 110 may bereduced by the first and second anti-reflection layers 130 and 160, anamount of light incident on the substrate 110 further increases.

The electron-hole pairs are separated into electrons and holes by thep-n junction between the substrate 110 and the emitter layer 120, andthe separated holes move to the p-type emitter layer 120 and theseparated electrons move to the n-type substrate 110. The holes movingto the emitter layer 120 move to the first electrodes 140, and theelectrons moving to the substrate 110 move to the second electrodes 170through the back surface field layer 150. Accordingly, when the firstelectrodes 140 of one solar cell are connected to the second electrodes170 of another solar cell adjacent to the one solar cell using electricwires (not shown), current flows through the cells and allows use of thecurrent for electric power.

The solar cell having the above-described configuration may be used in astate where the solar cell is positioned between a light transmissionfront substrate and a light transmission back substrate and is sealed bya protective layer.

An exemplary method for manufacturing the solar cell having theabove-described configuration is described below with reference to FIGS.3 to 6.

First, as shown in FIG. 6, with reference to FIGS. 3-5, a firstuniformly textured surface 111, an emitter layer 120, and a firstanti-reflection layer 130 may be formed at the front surface of thesubstrate 110. A second textured surface 113, a back surface field layer150, and a second anti-reflection layer 160 may be formed at the backsurface of the substrate 110.

Referring now to FIG. 6, a substrate 110 formed of the silicon wafer isgenerally manufactured by slicing a silicon block or an ingot using ablade or a multi-wire saw.

More specifically, a silicon wafer is provided and then is doped withimpurities of a group V element, for example, phosphorus (P) to form ann-type semiconductor substrate 110.

When the silicon block or the ingot is sliced, a mechanical damage layermay be formed in the silicon wafer. Thus, a wet etching process forremoving the mechanical damage layer may be performed, so as to preventa reduction in characteristics of the solar cell resulting from themechanical damage layer. An alkaline etchant or an acid etchant may beused in the wet etching process.

After the mechanical damage layer is removed, the wet etching process ora dry plasma etching process may be performed to form the first texturedsurface 111 in the front surface of the substrate 110 and the secondtextured surface 113 in the back surface of the substrate 110.

After the first and second textured surfaces 111 and 113 are formed, theback surface field layer 150 may be formed at each of the front surfaceand the back surface of the substrate 110 by doping each of the frontsurface and the back surface of the substrate 110 with impurities of agroup V element.

The second anti-reflection layer 160 formed of silicon nitride (SiNx:H)may be formed on the back surface of the back surface field layer 150 atthe back surface of the substrate 110.

Subsequently, an etched back process using the second anti-reflectionlayer 160 as a mask may be performed on the front surface of thesubstrate 110 to remove the back surface field layer 150 on the frontsurface of the substrate 110. The emitter layer 120 may be formed at thefront surface of the substrate 110 by doping the front surface of thesubstrate 110 with impurities of a group III element.

Subsequently, a natural oxide layer may be removed by etching thesubstrate 110 using hydrofluoric acid (HF), and the firstanti-reflection layer 130 may be formed on the emitter layer 120. Thefirst anti-reflection layer 130 may be formed by sequentially stackingthe aluminum oxide layer 133 and the silicon nitride layer 131. Thealuminum oxide layer 133 may serve as a passivation layer as well as ananti-reflection layer. The aluminum oxide layer 133 may be formed usinga plasma enhanced chemical vapor deposition (PECVD) method, a sputteringmethod, or other methods. A silicon oxide (SiOx) layer may be usedinstead of the aluminum oxide layer 133. The silicon nitride layer 131may be formed using the PECVD method, the sputtering method, or othermethods in the same manner as the aluminum oxide layer 133.

Next, the wet etching process or a dry etching process using a laser maybe performed to remove a portion of the first anti-reflection layer 130,thereby forming a plurality of first contact lines CL1.

After the plurality of first contact lines CL1 are formed, a conductivepaste obtained by mixing silver (Ag) with a glass frit may be printedusing an electrode pattern, dried, and fired.

When the conductive paste is fired, a punch through operation isgenerated because of lead (Pb) contained in the glass frit. Therefore,the second electrode 170 electrically and physically connected to theback surface field layer 150 may be formed.

After the second electrode 170 is formed, the first electrode 140 may beformed using a plating process. A method for forming the first electrode140 is described below.

The metal seed layer 141 may be formed on the entire surface of thefirst anti-reflection layer 130 and on the emitter layer 120 exposedthrough the first contact line CL1. The metal seed layer 141 may beformed by depositing nickel to a thickness of about 50 nm to 200 nmusing a vacuum method, for example, a sputtering method or an electronbeam evaporation method and then performing a thermal process at atemperature of about 300° C. to 600° C. in the nitrogen atmosphere.

Alternatively, the metal seed layer 141 may be formed by depositingnickel to a thickness of about 50 nm to 200 nm using an electrolessnickel plating process and then performing a thermal process at atemperature of about 300° C. to 600° C. in the nitrogen atmosphere.

According to the above-described process, the metal seed layer 141formed of nickel silicide (including Ni₂Si, NiSi, NiSi₂, etc.) isformed.

Next, the diffusion barrier layer 142 and the conductive layer 143 maybe sequentially formed on a portion of the metal seed layer 141. Morespecifically, a barrier layer may be formed on the metal seed layer 141,and an electroplating process is performed on the barrier layer, therebyforming the diffusion barrier layer 142 having a thickness of about 5 μmto 15 μm. A copper layer 143 a having a thickness of about 10 μm to 30μm and a tin layer 143 b having a thickness of about 5 μm to 15 μm maybe sequentially formed on the diffusion barrier layer 142.

Afterwards, the barrier layer may be removed, and then an etchingprocess using the tin layer 143 b as a mask may be performed to removean exposed area of the metal seed layer 141. Hence, the first electrode140 is formed.

A solar cell according to another exemplary embodiment of the inventionis described below with reference to FIGS. 7 to 9.

FIG. 7 is a schematic cross-sectional view of a solar cell according toanother exemplary embodiment of the invention. FIGS. 8 and 9 arecross-sectional views sequentially illustrating an exemplary method formanufacturing the solar cell shown in FIG. 7.

Since structure in a front surface of a substrate of the solar cellshown in FIG. 7 is substantially the same as the solar cell shown inFIG. 3, a further description may be briefly made or may be entirelyomitted and only structure in a back surface of the substrate of thesolar cell shown in FIG. 7 is described below.

A back surface field layer 150, a second anti-reflection layer 160, anda plurality of second electrodes 170 may be positioned at a back surfaceof a substrate 110.

The second electrodes 170 may be formed using a plating process in thesame manner as first electrodes 140 described above.

The second anti-reflection layer 160 may include a plurality of secondcontact lines CL2 exposing a portion of the back surface field layer150, so as to form the second electrodes 170.

The second contact line CL2 may have a width W2 (for example, a width ofabout 40 μm to 100 μm) greater than a width W1 of a first contact lineCL1, a plane area of the second contact line CL2 is about 5% to 15% of atotal plane area of the back surface field layer 150, and a pitchbetween the second electrodes 170 may be less than a pitch between thefirst electrodes 140, so that a line resistance is reduced.

Although it is not shown in detail, the second electrode 170 formedusing the plating process may be formed including at least one of ametal seed layer, a diffusion barrier layer, a copper layer, and a tinlayer, that may be sequentially stacked on the back surface field layer150 exposed through the second contact line CL2, in the same manner asthe first electrode 140 described above.

The solar cell having the above-described configuration may bemanufactured using the following exemplary method.

A process for respectively forming a first textured surface and a secondtextured surface on a front surface and a back surface of the substrate,a process for forming an emitter layer at a first textured surface ofthe front surface of the substrate and forming the back surface fieldlayer at a second textured surface of the back surface of the substrate,and a process for forming a first anti-reflection layer on a frontsurface of the emitter layer and forming a second anti-reflection layeron a back surface of the back surface field layer in an exemplary methodfor manufacturing the solar cell shown in FIG. 7 are substantially thesame as the exemplary method for manufacturing the solar cell shown inFIG. 6. Thus, a description will begin with the subsequent processes.

An emitter layer 120, a first anti-reflection layer 130, a back surfacefield layer 150, and a second anti-reflection layer 160 may be formed atthe substrate 110 having first and second textured surface 111 and 113.Then, a plurality of first contact lines CL1 may be formed in the firstanti-reflection layer 130, and a plurality of second contact lines CL2may be formed in the second anti-reflection layer 160.

The first contact lines CL1 and the second contact lines CL2 may beformed by performing a wet etching process or a dry etching processusing a laser to remove a portion of the first anti-reflection layer 130and a portion of the second anti-reflection layer 160.

When the first contact lines CL1 and the second contact lines CL2 areformed using the dry etching process using the laser, an etching processmay be performed using hydrofluoric acid (HF) to remove a damagedportion 121 of the emitter layer 120 and a damaged portion 151 of theback surface field layer 150 that may be damaged by the laser.

After the first and second contact lines CL1 and CL2 are formed, thefirst electrodes 140 and the second electrodes 170 may be formed using aplating process. A method for forming the first and second electrodes140 and 170 is described below.

The metal seed layer 141 may be formed on the entire surface of thefirst anti-reflection layer 130, the emitter layer 120 exposed throughthe first contact lines CL1, the entire surface of the secondanti-reflection layer 160, and the back surface field layer 150 exposedthrough the first contact lines CL1.

The metal seed layer 141 may be formed by depositing nickel to athickness of about 50 nm to 200 nm using a vacuum method, for example, asputtering method or an electron beam evaporation method and thenperforming a thermal process at a temperature of about 300° C. to 600°C. in a nitrogen atmosphere.

Alternatively, the metal seed layer 141 may be formed by depositingnickel to a thickness of about 50 nm to 200 nm using an electrolessnickel plating process and then performing a thermal process at atemperature of about 300° C. to 600° C. in a nitrogen atmosphere.

According to the above-described process, the metal seed layer 141formed of nickel silicide (including Ni₂Si, NiSi, NiSi₂, etc.) isformed.

Next, a diffusion barrier layer 142 and a conductive layer 143 may besequentially formed on a portion of the metal seed layer 141. Morespecifically, a barrier layer may be formed on the metal seed layer 141,and an electroplating process may be performed on the barrier layer,thereby forming the diffusion barrier layer 142 having a thickness ofabout 5 μm to 15 μm. A copper layer 143 a having a thickness of about 10μm to 30 μm and a tin layer 143 b having a thickness of about 5 μm to 15μm are sequentially formed on the diffusion barrier layer 142.

Afterwards, the barrier layer may be removed, and then an etchingprocess using the tin layer 143 b as a mask may be performed to removean exposed area of the metal seed layer 141. Hence, the first and secondelectrodes 140 and 170 are formed.

What is claimed is:
 1. A bifacial solar cell comprising: an n-typesubstrate including a first surface and a second surface opposite thefirst surface; an emitter layer positioned on the first surface of thesubstrate; a first anti-reflection layer positioned on the emitterlayer, the first anti-reflection layer including a plurality of firstcontact lines each having a first line width and exposing a portion ofthe emitter layer; a plurality of first finger electrodes electricallyconnected to the emitter layer exposed through a corresponding firstcontact line among the plurality of first contact lines, the pluralityof first finger electrodes formed in parallel with each other at a firstpitch; at least two first electrode current collectors each physicallyconnecting the plurality of first finger electrodes; a back surfacefield layer positioned on the second surface of the substrate; a secondanti-reflection layer positioned on a surface of the back surface fieldlayer, the second anti-reflection layer including a plurality of secondcontact lines each having a second line width larger than the first linewidth of each of the plurality of first contact lines and exposing aportion of the back surface field layer; a plurality of second fingerelectrodes each positioned on the second surface of the substratethrough a corresponding second contact line among the plurality ofsecond contact lines, the plurality of second finger electrodes formedin parallel with each other at a second pitch smaller than the firstpitch of the plurality of first finger electrodes; and at least twosecond electrode current collectors each physically connecting theplurality of second finger electrodes, wherein each of the plurality offirst finger electrodes and each of the plurality of second fingerelectrodes is formed of a nickel silicide layer directly contacting oneof the emitter layer or the back surface field layer, a nickel layer onthe nickel silicide layer, a copper layer on the nickel layer, and a Tinlayer on the copper layer, wherein the Tin layer is plated on top andside surfaces of the copper layer and does not physically contact thefirst anti-reflection layer, wherein a width of a top portion of each ofthe plurality of first finger electrodes, which is disposed on thecopper layer, is larger than a width of the corresponding first contactline, wherein a width of each of the plurality of the first fingerelectrodes is smaller than a width of each second finger electrode amongthe plurality of second finger electrodes, wherein a width of the nickelsilicide layer of each of the plurality of first finger electrodes isthe same as the first line width of each of the plurality of firstcontact lines, and a width of the nickel silicide layer of each of theplurality of second finger electrodes is the same as the second linewidth of each of the plurality of second contact lines, wherein thefirst anti-reflection layer includes a first layer on the emitter layerand a second layer on the first layer, and wherein a thickness of thenickel silicide layer is larger than a thickness of the first layer ofthe first anti-reflection layer.
 2. The bifacial solar cell of claim 1,wherein each of the plurality of first contact lines has the first widthof about 20 μm to 60 μm and each of the plurality of second contactlines has the second width of about 40 μm to 100 μm.
 3. The bifacialsolar cell of claim 2, wherein a plane area of each of the plurality offirst contact lines is about 2% to 6% of a plane area of the emitterlayer and a plane area of each of the plurality of second contact linesis about 5% to 15% of a plane area of the back surface field layer. 4.The bifacial solar cell of claim 1, wherein a thickness of each of theplurality of first finger electrodes is about 20 μm to 50 μm, thethickness of the first layer is equal to or less than 50 μm, and athickness of the second layer is about 50 nm to 100 nm.
 5. The bifacialsolar cell of claim 1, wherein the first surface and the second surfaceof the substrate are textured to form a first textured surface and asecond textured surface, respectively.
 6. The bifacial solar cell ofclaim 1, wherein the first anti-reflection layer includes a hydrogenatedsilicon nitride layer and an oxide layer positioned between the emitterlayer and the hydrogenated silicon nitride layer, the oxide layer beinga hydrogenated silicon oxide layer or an aluminum oxide layer, andwherein the second anti-reflection layer includes a hydrogenated siliconnitride layer.
 7. The bifacial solar cell of claim 1, wherein the secondanti-reflection layer includes a hydrogenated silicon nitride layer. 8.The bifacial solar cell of claim 1, wherein the nickel silicide layerhas a thickness of 50 nm to 200 nm.
 9. The bifacial solar cell of claim4, wherein each of the plurality of first finger electrodes has anaspect ratio of about 0.83 to 1, the aspect ratio being defined as aratio of a thickness to a width for each of the plurality of firstfinger electrodes, wherein a thickness of a projected portion of each ofthe plurality of first finger electrodes from a front surface of thefirst anti-reflection layer is greater than a thickness of an embeddedportion of each of the plurality of first finger electrodes from thefront surface of the first anti-reflection layer, and wherein theembedded portion of each of the plurality of first finger electrodes isformed of the nickel silicide layer, or is formed of the nickel silicidelayer and the nickel layer.
 10. The bifacial solar cell of claim 1,wherein a total plane area of the plurality of second contact lines islarger than a total plane area of the plurality of first contact lines.11. The bifacial solar cell of claim 1, wherein the plurality of firstfinger electrodes extend in a first direction and the at least two firstelectrode current collectors extend in a second direction crossing thefirst direction.
 12. The bifacial solar cell of claim 1, wherein a widthof each of the plurality of second finger electrodes is greater than awidth of each of the plurality of first finger electrodes.
 13. Thebifacial solar cell of claim 1, wherein a side surface of the firstanti-reflection layer and a side surface of the nickel silicide layercontact each other.
 14. The bifacial solar cell of claim 13, wherein thefirst anti-reflection layer extends under the Tin layer.